This invention relates to methods for adhering rubber to metal surfaces.
Reliable metal to rubber adhesion is required for many commercial and industrial parts. For example, rubber to metal adhesion is required in the automotive, medical, appliance and other industries where basic functions such as fluid control, energy conversion, sealing, vibration isolation and/or combinations of these functions are required. Additionally, tire to metal, metallic reinforcement of conveyor belts and hoses, and vibration dampening on motor and railroad mounts are further examples of industrial situations in which a variety of metals need to be adhesively bound to an associated rubber substrate.
Fluorosilicone elastomers have become increasingly popular due to their excellent high and low temperature performance. These elastomers also demonstrate advantageous resistance to fuels, oils, chemicals etc. due to the presence of the trifluoropropyl moiety in their repeat unit formula 
The polysiloxanes are crosslinked via pendent vinyl moieties by curing with peroxides. Compounds of fluorosilicones are formulated with reinforcing silica, various processing aids and other additives. These elastomers ar of special interest in the automotive industry where they can be used as conduits, valves or diaphragms and the like if properly adhesively bound to ancillary metal equipment.
It has however been difficult to bond these surfaces to metals, especially in those instances in which the adhesively bound surfaces are subjected to fuels, oils and other organic solvents.
Accordingly, it is an object of the present invention to provide methods for effectively adhesively bonding metal to rubber surfaces. It is an even more especially preferred object to provide a method for bonding fluorosilicone rubbers to a variety of metallurgies including, stainless steel, mild steel, brass, and aluminum.
These and other objects are met by the instant invention. Effective adherence of rubber, especially flourosilicone rubbers, to a variety of metal surface has been shown by use of an adhesive treatment comprising (I) an organofunctional silane and (II) a non-organofunctional silane.
The silanes (I) and (II) are partially hydrolyzed by addition thereof to an acidic aqueous or alcoholic medium. Solutions or dispersions of the silanes (I) and (II) are then applied to the requisite rubber or metal surface by dip coating, spraying, roller coating etc. After application of adhesive treatment to the surfaces, the surfaces may be blow dried or heated.
Although emphasis has been placed on effective adhesive bonding of fluorosilicone rubbers to a variety of metal surfaces, the adhesive treatment may also be used in conjunction with other rubber types such as EPDM, fluorocare rubber, and vinyl methyl silicone rubber. Tested metals with which the adhesive treatment has demonstrated efficacy include brass, stainless and mild steel and aluminum.
The invention will now be more specifically described in the following detailed description.
The adhesive treatment of the invention comprises use of an organofunctional silane (I) and a non-organofunctional silane (II).
I. Organofunctional Silane
This is a substituted silane compound having at least one free organofunctional moiety attached to an Si atom wherein the organofunctional moiety is adapted to react with the rubber substrate. More preferably, the organofunctional moiety is attached to one end of the Si atom with the remaining Si valences bonded to groups selected from C1-C6 alkoxy or acetoxy. More particularly, suitable organofunctional silane compounds can be represented by the formula 
wherein R is chosen from amino, C1-C6 alkylamino, vinyl, ureido, ureido substituted C1-C6 alkyl, epoxy, epoxy substituted C1-C6 alkyl, mercapto, mercapto substituted C1-C6 alkyl, cyanato, cyanato subsituted C1-C6 alkyl, methacrylato, methacrylato substituted C1-C6 alkyl, and vinyl benzyl moieties. The most preferred R substituent is vinyl. R1, R2, and R3 are independently selected from C1-C6 alkyl and acetyl groups.
Exemplary organofunctional compounds include xcex3-aminopropyltriethoxysilane (xcex3-APS); xcex3-mercaptopropyltrimethoxysilane (xcex3-MPS); xcex3-ureidopropyltrialkoxysilanes (xcex3-UPS); xcex3-glycidoxypropyltrimethoxysilane (xcex3-GPS); and a host of vinyl silanes (wherein R is vinyl). Most preferred are vinytrimethoxysilane, vinyltriethoxysilane and vinyltriacetoxysilane with vinyltrimethoxysilane (VS) most preferred.
II. Non-Organofunctional Silane
These are substituted silane compounds wherein one or a plurality of the Si valences are bonded to C1-C6 alkoxy and/or acetoxy groups. These may be represented by the formula II 
wherein m is 0 or 1; n is 0 or 1; and p is 0 or 1; with R4 selected from an aliphatic (saturated or unsaturated) group; aromatic group, or C1-C6 alkoxy or acetoxy; R5, R6, R7, R8, R9, and R10 may be the same or different and are chosen from C1-C6 alkoxy, H, or acetoxy; X, when present, is alkylene, alkenylene, phenylene or amino.
Exemplary non-organofunctional silanes include methyltrimethoxysilane (MS); propyltrimethoxysilane (PS); 1,2bis(triethoxysilyl)ethane (BTSE); bis(methyl diethoxysilyl)ethane (BDMSE); 1,2-bis(trimethoxysilyl)ethane (TMSE); 1,6-bis(trialkoxysilyl)hexanes; 1,2-bis-(triethoxysilyl)ethene; and 1,2-bis-(timethoxysilyipropyl) amine. Preferred are BTSE and BDMSE with BTSE most preferred.
The non-organofunctional silanes (I) and the non-organofunctional silanes (II) are both partially hydrolyzed by addition thereto into an aqueous/alcoholic solution including preferably a 40/60 vol % mix of ethanol/water. The silanes are added in an amount by volume of 0.5-10% based on the volume of the aqueous/alcoholic solution. Preferably, the silanes are present in an amount by volume of from about 1-5 vol %. Optimal adhesion has been shown when the pH of the solution is adjusted to between about 1-7. Most preferred is a pH of about 4.
The hydrolyzation of the silanes I and II is dependent on the pH of the solution. For example, acetic acid, oxalic acid and phosphoric acid may be mentioned as exemplary pH adjustment agents. Based on presently available data, it is preferred to use acetic acid as the pH adjustment agent.
Preliminary results indicate that the solution should be an ethanol/water solution with a 40/60 ethanol:water volumetric ratio being presently preferred.
The intended interfacial surfaces of the metal to rubber parts are contacted by the silanes (I) and (II) by dipping, spraying, painting etc. The following procedures were tried and found effective:
(1) blow dryingxe2x80x94the metal parts are dipped in the silane solution for at least 30 s and then are blown dry by filtered air;
(2) air dryingxe2x80x94the metal parts are dipped into the silane solution for at least 30 s and are allowed to dry for at least 30 min in the air;
(3) prebakingxe2x80x94the metal parts are dipped into the silane solution for at least 30 s and then dried in an air circulated oven for 20 min at 120xc2x0 C.;
(4) sprayingxe2x80x94the silane solution was sprayed on the metal parts and the parts were blown dry by filtered air; and
(5) preheated sprayingxe2x80x94the silane solution was sprayed onto the metal parts which were then preheated in an air circulated oven for 20 min and then blown dry by filtered air. In those situations in which brass metallurgy is to be adhesively bound to desired rubber, the blow drying and spraying techniques appear optimal.
When brass and fluorosilicone substrates are to be bonded, a mixture of VS and BTSE should be used in a single layer approach. A mixture of 3% VS and 2% BTSE in EtOH/H2O appears optimal in this situation.
Surprisingly it was found that a two step or two layer coating system appeared optimal for Al, SS and MS metals. That is, a first layer of non-organofunctional silane (II) is applied to the metal surface. Then, the organofunctional silane (I) is applied over the first layer as a second layer. This second layer consisting of the organofunctional silane (I) is placed adjacent the rubber surface for effective bonding. Of course the second layer could be placed over the rubber substrate with the metal and rubber surfaces then placed together and pressed to effect bonding.
Although applicants are not to be bound by any particular theory of operation, it is thought that the available organofunctional moiety on the silane effectively cross links with the rubber, probably to diene functions along the rubber macromolecule.
Based upon presently available data, the two step approach is preferred for Al, SS, and MSxe2x80x94rubber adhesion. The first layer solution should be 1% BTSE at pH 4 with the second layer solution being a VS 5% solution at pH 4. For brass, a single layer mixture of VS and BTSE is presently preferred. Both the VS and BTSE are commercially available from Witco""s Organosilicone group in Tarrytown, N.Y.
Although the present research indicates that the adhesive treatments of the present invention are effective for peroxide cured rubbers, it is thought that the invention shall also function for other curing systems such as nitro, quinone, azo, S, Se, bisphenol, diamine, Pt and Te curing systems.
Additional research has also shown that sandwiches of silicon wafers and siloxane rubber can be joined with the two step process and that these joints can successfully withstand etching with a 50% KOH solution at 80xc2x0 C.